Expand our studies with the domestic chick so that the relationship between changes in CNS excitability and learning-deficits and time of exposure to methyl mercury (critical period phenomena) could be supported by pharmacokinetic studies in the embryo and fetus. Since C14-radiolabeled methyl mercury became available, it was decided to use liquid scintillation spectrometry instead of wet chemistry described originally. This enabled us to inject the same doses of radioactive methyl mercury as that used for functional studies and examine the rate of appearance of methylmercury (as C14) in young whole embryo and parts of brain and other organs in older embryos, after injection into the yolk sac prior to incubation, at 7 days of embryogenesis and at 14 days of embryogenesis. Data indicate that exposure of the embryo or fetus via this route is almost immediate (at least in the older embryo) and concentrations achieved in the brains of 7-9 day old embryos in eggs injected prior to incubation are about 1/10 of that achieved in embryos injected on day 7 of incubation. This observation has much significance, since exposure of older embryos results in lower hatchability, indicating greater sensitivity of the older embryo to the embryolethal effects of methylmercury. When functional consequences of exposure to equieffective "killing" doses are compared, it turns out that exposure early in development renders the chick more sensitive to chemically and electrically induced seizures and less capable of learning behavior tasks when compared to controls or older embryos injected with the same (greater lethality) or lower (equal lethality) doses at a later stage of development. The fact that peak levels of methylmercury are achieved in the embryo soon after injection into the yolk, as well as the demonstration that equal brain concentrations on days 7-9 when injected prior to incubation or on day 7, do not produce equal functional alterations, points to a conclusion that a critical period exists during the first 1/3rd of embryogenesis for the observations made.